Definition Of Multicasting
A multicast message is, by definition, one packet in length. When a QP transmits a multicast message at the behest of a local application program, the packet's destination address defines to which ports (the multicast group members) the message is delivered.
As the packet arrives at each switch, the switch uses the packet's multicast DLID address to perform a lookup in its Multicast Forwarding Table (see “Multicast Forwarding Table Implemented” on page 675). The table lookup determines through which of the switch's exit ports (see Figure 22-1 on page 564) the packet is to be forwarded.
Figure 22-1. Switch Performing a Multicast
When the packet arrives at a CA port (see Figure 22-2 on page 565), it is delivered to all QPs within the CA that are members of the group.
Figure 22-2. Multicast UD Packet Arrives at a CA Port
If the packet should arrive at a router port, the router uses the packet's GRH:DGID address to perform a lookup in its routing table. There are two possibilities:
Link-Local Subnet ID. If the DGID address in the GRH is FF02:0:0:0:0:0:0:1, the packet is not to leave the subnet. This GID is used as the destination address when multicasting to the QPs that are members of the All CAs Multicast Group.
Other Subnets Participate In the Group. If the DGID address is a multicast GID and a lookup in the router's routing tables indicates that other subnets beyond this router are participating in the group represented by that GID, the router will pass the packet through one or more of its ports towards participating subnets.